(a) Fields of the Invention
The present invention relates to semiconductor devices employing a group-III nitride semiconductor. In particular, the present invention relates to power semiconductor devices of a group-III nitride semiconductor handling large electric power and having excellent heat radiation.
(b) Description of Related Art
Group-III nitride semiconductors refer to compound semiconductors which are made of a compound of nitrogen (N) and aluminum (Al), boron (B), gallium (Ga), or indium (In) represented by a general formula BwAlxGayInzN (w+x+y+z=1; 0≦w,x,y,z≦1).
The group-III nitride semiconductors have advantages such as wide band gap and high breakdown voltage associated with this gap, high saturation speed and high mobility of electrons, and high electron concentration in a heterojunction. Because of these advantages, research and development of the group-III nitride semiconductors is being carried out to apply them to power devices having high breakdown voltage and handling large electric power. In particular, as basic structures of the device shown above, use is made of heterojunction structures formed by stacking group-III nitride semiconductor layers with modified group-III element contents and different band gaps, or quantum well structures or superlattice structures formed by stacking the multiple heterojunction structures mentioned above. This is because with such structures, the modulation factor of the electron concentration in the device can be controlled.
FIG. 4 shows one of the most general forms of a conventional nitride semiconductor device employing a heterojunction (see, for example, Japanese Unexamined Patent Publication No. 2002-16245 or U.S. Pat. No. 6,316,793). In FIG. 4, an operating layer 12 of gallium nitride (GaN) and a barrier layer 13 of aluminum gallium nitride (AlxGa1-xN) are sequentially stacked on a substrate 11. A heterojunction is formed at the interface at which the operating layer 12 and the barrier layer 13 with different band gaps are stacked.
A source electrode 14, a drain electrode 15, and a gate electrode 16 are formed on the barrier layer 13, and these electrodes operate as a heterojunction field effect transistor (abbreviated hereinafter to an HFET). The gate electrode 16 and the barrier layer 13 form a Schottky barrier. At the heterojunction interface between the barrier layer 13 and the operating layer 12, electrons are accumulated at a high concentration. The accumulated electrons result from the difference of the amount of the spontaneous polarization and the difference of the amount of the piezo-polarization between the barrier layer 13 and the operating layer 12, n-type impurities doped into the barrier layer 13 as appropriate, and other uncontrollable defects in the semiconductor layers, and they form a 2-dimensional electron gas (2DEG), which in turn acts as channel carriers for the field effect transistor. Typically, silicon nitride (SiN) is used as a surface passivation film 17.
As a semiconductor device of a group-III nitride semiconductor aiming at a higher breakdown voltage, the following structure is described (see, for example, Japanese Unexamined Patent Publication No. 2004-363563). This structure is characterized in that it includes: a conductive layer; an operating layer made of a group-III nitride semiconductor and formed above the conductive layer, a barrier layer made of a group-III nitride semiconductor and formed on the operating layer; a first source electrode, a drain electrode, and a gate electrode formed above respective portions of the barrier layer; a second source electrode connected to the first source electrode; and an interconnection member connecting the first source electrode and the conductive layer through a via hole penetrating the channel layer and the barrier layer.
In order to apply semiconductor devices using a group-III nitride semiconductor to power devices with high breakdown voltage, it is necessary to take measures to effectively radiate heat generated from the semiconductor device to the outside thereof. This is because the higher voltage and the larger current an application has, the greater amount of power is consumed in and the greater amount of heat is generated from the semiconductor device. In particular, for HFETs of multifinger structures commonly employed as power semiconductor devices handling large electric power, because of their structures, heat generated in the operating layer stays ununiformly inside the device. In order to provide a uniform heat profile inside the semiconductor device, it is conceivable that a heat radiating component referred to as a radiation fin, a heat spreader, or the like is mounted on the semiconductor device. The use of such a heat radiating component, however, requires a space enough to accommodate the component, which goes against the trend toward savings in the device space. Therefore, a desirable power device is a semiconductor device which has a space-saving structure and can sufficiently exert the effect of heat uniformization.